Image Cache Support for localdisk driver¶
https://blueprints.launchpad.net/nova-powervm/+spec/image-cache-powervm
The image cache allows for a nova driver to pull an image from glance once, then use a local copy of that image for future VM creation. This saves bandwidth between the compute host and glance. It also improves VM deployment speed and reduces the stress on the overall infrastructure.
Problem description¶
Deploy times on PowerVM can be high when using the localdisk driver. This is partially due to not having linked clones. The image cache offers a way to reduce those deploy times by transferring the image to the host once, and then subsequent deploys will reuse that image rather than streaming from glance.
There are complexities with this of course. The cached images take up disk space, but the overall image cache from core Nova takes that into account. The value of using the nova image cache design is that it has hooks in the code to help solve these problems.
Use Cases¶
- As an end user, subsequent deploys of the same image should go faster
Proposed change¶
Create a subclass of nova.virt.imagecache.ImageManager in the nova-powervm project. It should implement the necessary methods of the cache:
- _scan_base_images
- _age_and_verify_cached_images
- _get_base
- update
The nova-powervm driver will need to be updated to utilize the cache. This includes:
- Implementing the manage_image_cache method
- Adding the has_imagecache capability
The localdisk driver within nova-powervm will be updated to have the following logic. It will check the volume group backing the instance. If the volume group has a disk with the name ‘i_<partial uuid of image>’, it will simply copy that disk into a new disk named after the UUID of the instance. Otherwise, it will create a disk with the name ‘i_<partial uuid of image>’ that contains the image.
The image cache manager’s purpose is simply to clean out old images that are not needed by any instances anymore.
Further extension, not part of this blueprint, can be done to manage overall disk space in the volume group to make sure that the image cache is not overwhelming the backing disks.
Alternatives¶
- Leave as is, all deploys potentially slow
- Implement support for linked clones. This is an eventual goal, but the image cache is still needed in this case as it will also manage the root disk image.
Security impact¶
None
End user impact¶
None
Performance Impact¶
Performance of subsequent deploys of the same image should be faster. The deploys will have improved image copy times and reduced network bandwidth requirements.
Performance of single deploys using different images will be slower.
Deployer impact¶
This change will take effect without any deployer impact immediately after merging. The deployer will not need to take any specific upgrade actions to make use of it; however the deployer may need to tune the image cache to make sure it is not using too much disk space.
A conf option may be added to force the image cache off if deemed necessary. This will be based off of operator feedback in the event that we need a way to reduce disk usage.
Developer impact¶
None
Implementation¶
Assignee(s)¶
- Primary assignee:
- tjakobs
- Other contributors:
- None
Work Items¶
- Implement the image cache code for the PowerVM driver
- Include support for the image cache in the PowerVM driver. Tolerate it for other disk drivers, such as SSP.
Dependencies¶
None
Testing¶
- Unit tests for all code
- Deployment tests in local environments to verify speed increases
Documentation Impact¶
The deployer docs will be updated to reflect this.
References¶
None
